1. Field of Invention
This invention relates to a method of automatically recognizing a subdivision pattern of radiation images stored on a stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to a stimulating ray beam such as a laser beam which causes the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals, and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, in order to eliminate various influences caused by variations in image recording conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is desired to ascertain such image input conditions of the radiation image stored on the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the object (e.g. the chest or the abdomen of the human body) or the image recording method used, such as plain image recording or contrasted image recording, before reproducing the radiation image to a visible image, and then to adjust the read-out gain to an appropriate value based on the ascertained image input conditions or the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically. It is also desired to adjust the scale factor to optimize the resolution in accordance with the contrast of the image input pattern.
Ascertaining of the image input information may be carried out prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, a read-out operation for ascertaining the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, the readout gain and/or the scale factor is adjusted to an appropriate value, and/or an appropriate signal processing is conducted, on the basis of the image input information obtained by the preliminary read-out.
Various methods may be used for approximately ascertaining the image input information on the stimulable phosphor sheet from preliminary read-out image signals obtained by the aforesaid preliminary read-out. One method is to utilize a histogram of the preliminary read-out image signals. Specifically, the image input information can be ascertained from, for example, a characteristic value such as the maximum signal value, the minimum signal value, or a signal value at a point where the frequency is the maximum in the histogram. Therefore, a visible radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, can be reproduced by adjusting the read-out conditions such as the read-out gain and the scale factor and/or the image processing conditions on the basis of the histogram.
On the other hand, in the course of radiation image recording, it is often desired that portions of the object not related to diagnosis or the like be prevented from exposure to a radiation. Further, when the object portions not related to diagnosis or the like are exposed to a radiation, the radiation is scattered by such portions to the portion related to diagnosis or the like, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, the irradiation field is limited to an area smaller than the overall recording region on the stimulable phosphor sheet when a radiation image is recorded.
However, in the case where the image input information stored on the stimulable phosphor sheet is ascertained in the manner as mentioned above, the problem as described below arises. When an irradiation field is limited to an area smaller than the image recording region on the stimulable phosphor sheet and the preliminary readout is carried out over an area markedly larger than the irradiation field, for example, over the overall image recording region on the stimulable phosphor sheet, the image input information actually stored within the irradiation field is ascertained incorrectly. Specifically, in the aforesaid case, since the preliminary read-out image signals at regions outside of the irradiation field are also included in the histogram, the histogram does not accurately represents the actual image input information stored within the irradiation field.
The applicant have proposed various methods of recognizing an irradiation field as disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-39039. The aforesaid problem can be eliminated by automatically recognizing the irradiation field by use of the proposed methods, and carrying out the preliminary read-out only for the region thus recognized.
On the other hand, in the course of radiation image recording on the stimulable phosphor sheet, subdivision image recording is often carried out. In subdivision image recording, the recording region on the stimulable phosphor sheet is divided into a plurality of predetermined subdivisions, and the respective subdivisions are exposed to radiation for image recording. Subdivision image recording is economical since, for example, when an image of a small object portion is recorded on a large stimulable phosphor sheet, images of a plurality of object portions may be recorded on a single stimulable phosphor sheet. Also, the radiation image recording and read-out processing speed becomes high.
However, in the case where irradiation fields are limited when subdivision image recording as mentioned above is carried out, the respective irradiation fields become separated from each other. FIG. 2 shows the recording condition on a single stimulable phosphor sheet 103 in the case where subdivision image recording is carried out by dividing the recording region on the stimulable phosphor sheet 103 into two subdivisions, and the irradiation field is limited in each subdivision image recording step. In FIG. 2, B1 and B2 denote the respective irradiation fields. In most conventional methods of recognizing an irradiation field, recognition of the irradiation field is carried out on the assumption that a single irradiation field is present on a single stimulable phosphor sheet. With such methods, the irradiation fields as shown in FIG. 2 are recognized incorrectly. On the other hand, a method of automatically recognizing a plurality of irradiation fields on a single stimulable phosphor sheet has also been proposed. However, with the proposed method, the algorithm for recognition of irradiation fields becomes very complicated, and a very expensive apparatus is necessary for executing the method.
In the case where information on the positions of the respective subdivisions are instructed by manually entering the information representing a subdivision pattern on the stimulable phosphor sheet to an irradiation field recognizing apparatus in the course of recognition of irradiation fields, processing for detecting a single irradiation field in each subdivision may be carried out, and the problem that the algorithm for recognition of the irradiation fields becomes very complicated can be eliminated. However, it is very troublesome to manually enter the subdivision pattern each time radiation image read-out from the stimulable phosphor sheet is to be carried out.